Method and apparatus for sifting



July 24, 1962 Filed Nov. 4, 1959 c. w. WARD 3,045,817

METHOD AND APPARATUS FOR SIFTING 4 Sheets-Sheet 1 INVENTOR CHARLES WA D ATTORNEY July 24, 1962 c. w. WARD 3,045,817

METHOD AND APPARATUS FOR SIFTING Filed Nov. 4, 1959 4 Sheets-Sheet 2 I 27 INVENTOR CHARLES w. WARD 3I ATTORNEY July 24, 1962 c. w. WARD 3,045,817

METHOD AND APPARATUS FOR SIFTING Filed Nov. 4, 1959 SheetsSheet 3 I I-II 9' I [In 3/ AUDIO OSCILLATOR INVENTOR CHARLES W- WARD ly/mu} (1 ATTORNEY July 24, 1962 c. w. WARD 3,045,817

METHOD AND APPARATUS FOR SIFTING Filed Nov. 4, 1959 4 Sheets-Sheet 4 AUDIO OSCILLATOR POWER AMP.

Q mmmmm a IQ ///////Z777T//// iNVENTOR CHARLES W WARD ATTORNEY United States Patent ()fiice 3,945,817 Patented July 24, 19-52 The present invention relates to a sifting method and an apparatus therefor, and in which the apparatus may em body one or more screens or sieves for separating a particulate material into selected size gradations and which screen or screens are disposed in a chamber communicating with an oscillatory diaphragmarrauged to cause a corresponding oscillating air motion at the vicinity of the relatively stationary screen or screens to impart movement to particles being sifted, whereby sifting is accomplished by means of a cyclic motion of the particles.

In many of the process industries satisfactory production is dependent upon proper selection of particle size of various ingredients used as raw materials. This requires careful selection and grading by screening or sifting processes, and consequently, the sifting process step or steps provide a very important factor in calculating cost of preparation, because of the time and equipment involved, in addition to equipment replacement costs. Size control is particularly important in the preparation of ceramic materials, and particularly those wherein electrical dielectric characteristics are critical. Particular emphasis is placed upon size control of raw materials in the newly developing field of ferromagnetic compositions known as ferrites and in other sintered materials involving semiconductor application. Sitting becomes a particularly burdensome problem in processes using materials such as spray-dry powders which tend to cling to the screen, and in processes wherein the materials include considerable quantities of fines after they have been crushed. Fines are difficult to remove from the particles and they also tend to clog the meshes of the sieves.

Various elforts have been made in the past to reduce the time for sifting without impairing the desired results. Sifting on a commercial scale has had several unsatisfactory aspects which cause it to be a relatively inefficient operation. In certain installations stationary screens have been used without additional auxiliary apparatus for promoting sifting. These are quite low in eihciency and capacity in the finer sizes of screens, with a great tendency to bind and require a large amount of head room for their operation. Stationary screens do have the advantage of low initial and repair costs and ease and speed of installation. Although the present invention contemplates the use of stationary screens, it utilizes their advantages, and at the same time, circumvents the above-mentioned disadvantages.

In an effort to increase the efiiciency of screening or sifting techniques, sifting devices have been provided which impart a relative motion to the screen or other separating element. These devices have included revolving screens, such as simple, compound and conical, and prismatic trommels have been provided, which have particular application in flour mills. The revolving screens later were improved by oscillating screens which afforded higher capacity, and by shaking the screens which were fairly etiicient in sizes coarser than one-half inch, but in finer sizes the capacity was low if high efficiency was desired. The chief disadvantage of shaking the screen is high cost of maintenance both of the screen and the supporting structure.

Where large capacity and high efficiency were desired in the past, the use of vibrating and oscillating screens was standard practice. The capacity, especially in the finer sizes was much greater than other screens and materially reduced the grinding effect. However, there was vconsiderable wear of the screen material, the supporting structure and of the various parts providing the oscillation or vibration to the screens. in fact, where it is desired to provide exacting size control, it becomes particularly important to replace vibrating screens quite often to insure exact mesh size.

In the case of testing or control wherein the specifications call for definite sizes of material, it has been the conventional practice :to use devices which consist of a receptacle to hold a series of superimposed sieves of standard mesh size and a motor driven shaker which imparts to the sieves both a circular and a tappingmotionp The tapping motion is imparted to a cover placed over the series of sieves and there is a tendency to cause the super imposed sieves to stick to one another, necessitating considerable time for their separation, and very often caus ing damage to the sieves by the use of tools for prying them apart Sitting devices of this nature also have been found to actually force particles through [the meshes by the tapping action, and have permitted particles to pass through a particular screen size which ordinarily would not pass a stationary screen unless forced therethrough. In certain cases relatively elongated particles have also been found to pass through these sieves which obviously tend to negative the results when used for standardization purposes. This has been verified from the use of glass beads which were supplied as standards to be used for control purposes. A number of the beads, upon resifting in a device of the present invention, were found :to have been fused together, providing a relatively elongated structure, which, in a particular size range, would ordinarily" not have passed through the sieve of a particular size unless forced therethrough. Thus, standardization control samples which were supplied for purposes of standardization were found not to be a true standard when closely examined, whereas the apparatus of the present invention has successfully provided sample after sample of standard size particles, without any indication of weight variation of graded sample amounts between s'iftings.

The present invention contemplates the use of one or more screens or sieves disposed with their respective mesh openings transversely of an oscillating column of air passing therethrough. The screen or screens are preferably stationarily mounted relative to their support and are not caused to vibrate, oscillate or rotate in any manner relative to the stream or the sifted particles. The air column is preferably provided by means such as a conventional audio speaker driven by a power amplifier and an audio oscillator circuit of conventional construction. The circuits may be of fixed power and oscillation for production purposes wherein the sifted materials are always to be of the same composition and are to be sifted to the same size. However, for broadest operation, it is desirable to provide variable power amplifiers and audio oscillators to permit greater versatility.

It is an object of the present invention to provide an improved method of sifting particulate material, wherein the material is spread uniformly over the surface of a sieve, subjecting the material and sieve to a transversely disposed column of reciprocably moving air at desired cyclic intervals and with sufiicient energy to raise the material from the sieve as the air moves towards the material from below the sieve and then to urge the material in a direction towards the screen as the direction of air movement is reversed.

It is another object of the present invention to provide an improved sifting apparatus including a sieve or sieves for screening or sifting a particulate material by means of subjecting the material to be screened to the action of an oscillating air column transversely of the sieve or sieves.

It is a further object of the present invention to provide a sifting apparatus which includes a sieve or sieves "sieves of lesser'mesh size.

ofFI Ggl FIG '3 is a cross-sectional view of the apparatus taken in the plane oflines 3-3 of'FIG. 2'and particularly il- 'lustrating' the preferred'relationship of the with the series of superimposed sieves;

' proved separatory grading; and. sifting device.

3 disposed ina chamber communicating diaphragm arranged to cause a correspondingoscillating air motion at;the;vicinity of'the relatively stationary sieve or sieves. to impart movement to particles being sifted and which-includes means for providing cyclicmotion tothe oscillating diaphragm.

It is a more specific object of the present invention to provide an improved'sif ting apparatus comprisingasieve or series of superimposed sieves disposed in'a'chamber communicating at one end with an oscillatory diaphragm and an electromagnetic vibrator operatively associated I with the diaphragm to impart oscillatory motion thereto, and to thereby impart cyclic motion to air or other gas residing in said column to'impart acyclic motion to the particulate material to be sifted. v

7 It is another specific object of the present invention to I, ,provide' a specific embodiment of the sifting apparatus which includes a pair of electromagnetic vibrators supplied'from a common energy source comprising a con- 'ventional audio oscillator and power amplifier and with the'respective polarity of'th'e core of each vibrator adjusted to provide alternate reciprocal motion to the oscillatory diaphragrns respectively'driven thereby, wherein the apparatus may be adapted for simultaneous sifting of material into two size gradations and wherein the chambermay be sealed frorn atmosphere and containa gas which is inertfto reaction with the sifting material.

with an oscillating I slidable sieve supporting member" 1 4, by means of op-. I

posed channel members 15. Handles ordrawer. pulls 16 and 17 are respectively provided for the drawer 12 and the sliding sieve support 14.", It is to be noted that. the

present construction provides a gap or air space 18 between the drawer 12 and the member 14 for purposes hereinaftcr described.

As shown in FIG. 2, tubular upright members Hand 20 are oppositely disposcd'from one another at'either, side of the base plate 10, and'are fastened by means of welding or other suitable means to "the side panels 11. The members 19 and 20 extend upwardly of the base to provide a' slidable support for a conventional electromag netic speaker 25, such as those conventionally used in The speaker .25 is mounted coaxially to a similarly The apparatus of the present invention is adaptable to a-production sifting mechanism, wherein a single screen is used toprovide a desired size range, and also for use in a standardizing or separating apparatus for providing 7 the separation of a given amount of material of conglomerate' size mixture into various sieve sizes by means of conventional superimposed sieves, each having a standardme'sh, thezlargest mesh being'uppermost relative'to I With reference to the drawings? FIG. lis a side elevational view, partially in section,

' of an apparatus embodying the present invention, wherein a series of superimposed sieves of varying mesh are used for separation of a particulate material into various size gradations; v Y

FIG. 2 is a vertical sectional. view taken on lines 2'-2 FIG. 4 is aside elevational view ofa production type embodiment ofthe present invention;

16. 5 is a vertical sectional view taken, along lines speaker done configured opening 27 in a speaker-support member 28, which member may be of plywood or the like. The member 28 is 'slida-bly fastened to the tubular support members 192and by means of laterally extending guide members 29. The opening 27 is preferably covered at the bottom of the support 28 by means of a flexible membrane 30, which may he rubber or similar material. The membrane is principally used forprotection of the components of the speaker from contact with dust or fines evolved during the screening and separating process, as hereinafter described. It is preferable to totally enclose the speaker as shown, by means of flexible side.

panels'31, in order to prevent dust from entering into the components of the speaker. The side panels. 31 are supported between the lower support member 28 and an upper support member 32, which may also be slidably fastened to the upright support members 19 and 20 by means of laterally extending guide member 33. The entire enclosure'is held in spaced relationship by means of the corner straps 34/ Supported from the top of the vertical uprights 19 and 20 is an air cylinder 40 having 7 a reciprocating plunger (not shown) and attached to a Y FIG. 6 is a schematic circuit diagram illustrating the a V supply circuit for the air column vibrating mechanism;

''FIG. 7 is a sectional view taken one planecomparable to the plane of FIG. 5'of still-another embodiment of the present inyention; 5 v a FIG. 8 is a schematic circuit diagram illustrating a supply circuit for the embodiment of FIG. 7.

V j Referring in particular to the embodiment .of FIGS.

1 3,'it'will be apparent that the apparatus of the present invention maybe adapted for use in conjunction with a stack-of superimposed cylindrical sieves toprovide an im- The stacked sieves may be removably disposed in. the apparatus. for lateral -withdrawal and replacement, without requiring'separate hand removal operations of fastening 7 nuts, bolts or other clamping means. i As illustrated, the apparatus includes a supporting base comprising a base plate 10 secured at each side. to vertically disposed panels 11 to provide opposed open ends. The upwardlyprojecting side' panels 11 and the baseplatejlt) provide support for a removable drawer member 12 resting, as shown in FIG. Lagainst a rear stop member 13. The upwardly projectin'g portions of the panels 11 further support a laterally ,7 conventional valve (not shown) arranged for communication with an air supply to provide a means of alternatively raising and lowering the speakerunit from and to the position shown in phantom lines of FIG; 1. a The unit may be moved between the respective ends of the upright support members 19 and 20 to be disposed at any desired position therebetween. p Y

The speaker 25 is of conventional design, and provides an oscillatory conical diaphragm 41 mounted in aprotec tive support 42, including spaced openings 42a, and may include a reinforcing inner diaphragm portion 43." The conical diaphragm is secured at its inner end to the arma V ture 44 associated with an electromagnetic core 45. The conventional moving coil and magnetizing coil are not specifically illustrated, but complete the usual operating I circuit components of the speaker unit in a conventional manner.

. The sieves 26 of-the present embodiment are of conventional construction and are placed inthe usual manner V with the sieve of larger mesh opening being placed upper most' of the stack. Such arrangement permits separation of the material into lesser sizes downwardly relative thereto. These sieves are disposed substantially coaxial'of an 7 opening 48' in the sieve support member 14 by inserting the ring 46 ofthe lowermost sieve member 26 inwardly of the opening 48 and supporting the same by means of the radially extending flange 47, and then stacking the desired number of sieves in superimposed telescoped relation thereon.

In order to sift, the slidable sieve support 14 is'mov'ed 1 laterally outwardly to the phantom position of 'FIG. 1- 1 and the particulate material to be sifted is spread evenly over the surface of the screen of the uppermost sieve 26 to take full advantage of the area provided and to provide full effect of the air acting thereon. The member 14 may then be moved inwardly while the speaker unit is in its upper (phantom view) position. The speaker cone 25 is then brought into the solid line position, with its membrane 30 resting upon the upper edge of the uppermost sieve 26 of the stack. The membrane 30 will also act as a sealing gasket between the uppermost sieve and the speaker 25. The speaker 25 is electrically connected to a conventional audio oscillator and power amplifier as illustrated diagrammatically in PEG. 6. The oscillator and power circuits are energized from their respective power supply and tuned to the desired frequency and amplitude.

It will be apparent that after the cone, or diaphragm 41 of the speaker 25 is set into vibration that it will cause the column of air existing in the chamber enclosed by the diaphragm to be reciprocably moved. The air gap 18 is provided as a means of permitting air to be moved inwardly and outwardly and to prevent dampening of the speaker caused by an entirely closed column. The screens or sieves 26 remain relatively stationary with respect to the air column movement and the only action provided to encourage sifting is caused by the movement of the air induced by the properly tuned vibration of the speaker. It has been found to be quite satisfactory in the case of sifting glass beads between 30 and 100 mesh screen size to provide a power input of 25 watts to the speaker 25, at a frequency range of between 25 and 50 cycles per second. A screen diameter of 8 inches and a standard 12 inch speaker was used to completely sift 200 grams of material into 40 grams each of 30, 40, 60, 80 and 100 standard mesh glass bead size in 30 seconds. Whereas, the same amount took mrnost five minutes to provide equivalent sifting in a conventional rotating sieve subjected to periodic tapping.

If the frequency is increased beyond the desired frequency of 45 cycles per second, a granular material tends to cake and obstruct at least certain portions of the sieve.

Too low a frequency may permit air to pass through of the device, a sample of ordinmy wheat flour was sifted through a 325 mesh screen, in which the mesh opening is 0.0017 inches, whereas flour has not heretofore been effectively sifted by the usual circulating or oscillating screens, even with the tapping feature.

To illustrate the effectiveness and accuracy of sifting by the present apparatus, standard glass beads specified as US. Standard Sieve Size, or Tyler equivalent, were purchased in No. 30, No. 40, No. 60, No. 80 and No. 100 sieve sizes for purposes of evaluation. The glass beads provide a satisfactory standard as they do not have the usual difliculties associated with materials having fines and a tendency to cling to one another, as in the case of crushed clay, talc or the like. Forty grams each of the respective bead sizes were thoroughly mixed together andplaced in a conventional rotating and tapping device.

As a comparison, a standards evaluation was made of the present embodiment using glass beads of standard size, which the supplier stated were prepared in accordance with US. Bureau of Standards accepted techniques. Beads of the various standard sizes were accurately weighed into 40 gram samples of each size and were thoroughly mixed and spread evenly over the surface of the uppermost sieve. The speaker used included a /2 inch diameter cone, and was provided with a 25 watt input at a frequency of 32 cycles per second. The Table I, hereinbelow, sets forth the respective weights sifted for 30. second and 1 minute operations, with and without the 6 rubber membrane 33 on the speaker 25. It will be apparent from the results that the membrane has little or no effect on the operation of the device, but merely acts as a dust protector for the speaker components.

TABLE I Standards Evaluation Without Rubber With Rubber IVIembrane Membrane Standard Sieve Size 30 Seconds 1.0 Minute 30 Second 1.0 Minute Operation, Operation, Operation, Operation, gms. gms. gins.

Total 199. 6 199. 6 199. 6 199. 6

It will be observed from Table I that the variation between the 30 second operation and 1 minute is negligible in amount sifted. Similar tests made on conventional apparatus in accordance with the technique set out in the US. Bureau of Standard Procedures Powder Sifting, as outlined in the Chemical Engineers Handbook, 1952 edition, pages 962-964, took five minutes or more to obtain results not nearly as consistent. It was also found that the beads, which were supplied as being standard, were not, in fact, of uniform size, as there were a number of elongated particles which appeared to be the result of fusing together two or more glass particles. The elongated particles pass through the standard sieves under the conventional test techniques and apparatus, but were segregated by the apparatus of the present invention, to provide a true control standard.

High speed, magnified movies taken at the rate of 2000 frames per second for about 1.5 second period clearly illustrated, when projected at normal speed, that sifting was substantially completed of a pre-Weighed amount of mesh size material mixed with larger sized material through an 80 mesh screen within this short period. The air column tended to lift the particlesfrom the screen and alternatively permit the appropriate size to pass through the mesh opening without visible vibration of the screen.

The apparatus and sifting method in accordance with the present invention is especially useful for the separation and screening of compositions having relatively large amounts of fines caused by crushing and other preparatory operations, such as raw materials for ceramic and ferrite production. The usual sifting techniques in these industries require batch handling in devices having vibrating screens, which are replaceable at relatively short intervals, and which techniques are also very time consuming.

To illustrate another important application of the present method and apparatus, at times it becomes important to screen or sift particulate matter having prop erties not conducive to conventional screening techniques. For instance, in certain applications of epoxy resins sizing of particles becomes important. Heretofore, certain of the epoxys could not be satisfactorily screened as it becomes tacky at room temperature under-even slightly humid conditions. In addition, the material exhibits a balling or rolling effect upon the surface of conventional rotating screens. What little material does go through the mesh openings, tends to clog the openings because of its sticky nature.

The method and apparatus of the present invention provide a very convenient means of sifting materials even of the nature of the above-mentioned epoxy resins. The resin was sifted in the apparatus of the embodiment of FIGS. 1, 2 and 3, using a series of screens comprising Nos. 30, 40, 60, 80 and mesh screens. A 400 gram sample was sifted through this stack in 30 seconds or files.

FIGS; 4 and'S, wherein a removable sieve'50 is mounted 5.

in a housing 51 and adapted to be interchangeable with sieves of diflierentme'shSize; Like reference numerals denote like parts throughout the several embodiments In production facilities,'it isthe'bsual practice to sift the material through a single desired screen size, as it is recovered fora specific purpose. IThe housing 51 in; Vcludes a slidable product drawer-52 for removal of the {sifted material of desired size.. Another very important aspect' discovered during investigation and trial of the present device, is that as. the speaker moves the 'air l column .inlr'eciprocal fashion, the fines denoted by the i reference numeral 5 3, appear to be sepafatedfat the very "beginning of operation. This provides immediate SGPBI'Q'.

'tion or the fines "53 from the unsifted raw material 54 alrnost simultaneously with its entry from the hopper 55. 29

Thus, two separate pilesjof sifted material are fornied for added'ease in separation-a pile of fines 53 directly below the entrance opening 57and a pileof sifted material 56; V'I'he screen 50 is preferably placed at an angle 1 to -permit the oversize particles to run 013? at the exit open- '25 ing 59 under the influence of gravity. A. satisfactory means-for tilting the apparatus for-this purpose is provided by adjustable legs 60. The screen 50, as aforementioned, is adapted "for r eplacement and interchangeability,

'and lock inte position by means of the cammedsurface clamping members 61 rotatably mounted on the housing 51and arranged for frictionally gripping the" screen 50.:

"In certain cases it has been foundto be more eflicient' Y to provide twin speaker cones .70 and 71 as shown in the a I embodiment 'of FIGS. 7 and 8, the action ofwhich'may bear-ranged to permit a' sealed housing, if desired. The

speakers 70 and .71 are each connected to a power amplifier and audio oscillator, as in the case of a single speaker installation, and as specifically illustr'atedin FIG.

8. 'I'he-polarity of each speaker is adjusted for alterr 'nating cyclic operation. The material to-be sifted isfed through the separate housing portions 72 and'73 respecit tiv elyincludingsieves'74 and 75. The sieves may be of the-same or different mesh size.

The'present embodiment prov1des 'a versatile unit' of increased capacity from the twin speakers acting upon air :4

or other gas in the chambers 72 and 73; The present structure maybe totally enclosed as" there is no necessity 'for air .to :be' permitted access and egres's,as it will merely be shuttled back and forth between the opposed chambers V f 72 and 73 throughthe chamber 76.. Sliding drawers 77 a'nd'78 are disposed in' the chamber 76 toreceive sifted material inrthe same manner as the drawers of theme-- j viously described embodiments. The present embodie I r V frnentthus permits an entirely enclosed 'structure'in cases 1 "where a dust-free atmosphere is essential to ancillary op 7 *eratio'ns'or procedures. In'addition; the enclosure may a be filled withinert gases, in cases "where airJmight 'afiect the particular material to be sifted. r

In each of the illustrated embodiments the power 'and frequency-may be varied toisuit the' sifting material, ac-

- a ,cording to its size and weight. From the aforementioned w:high "speed movingpictures, it appears that only power suflicient to motivate the particular particles and to raise them above lthetscreen level for purposes, offpermit'ting smaller particles to pass therethrough. The smaller parti cles appear to pass through thescreen mesh due'tov a combination ,of gravitational "action and "action of the 1 reciprocating air flow as it is moved in a direction towards the s creeing It is 'also apparent that ahteadyistreambf particles to fallt'hrough the mesh, rather than totbe forced pwardly under fluid pressure in'a steady stream. The V 7 it "mits the particles to have .accessythrough the: openings, 'the larger'particles being forcibly' moved upwardly away In most conventional sifting devices it is necessary to clean the screens by means of a wire or other brush to remove particles which 'have become lodged inthe mesh I openings; Very often the particles are securely wedged because of being subjected to pounding action, causing undue wear and abrasion by brushing of relatively fine. mesh screens. The screens, when used in the present apparatus, may be cleaned in most cases by simply inverting themtand gently-tapping the supporting edges upon a stationary object. Rarely has it been found that particles have becomeiso tightly wedged in the meshes that brush 5 ing was necessary to dislodge them. a a I 'Investigationhas further established that the speaker may be transversely disposed at either the top 'or bottom end of'the housing, oppositely spaced'from thescreen or sieve, and work effectivelyr It will be apparent, however, that when disposed under the screen (not shown),

effect the operating efliciency of the vibrating member. It is to be understoodthat the term airi as used any'gaseousratmosphere capable of providing the results adverse efiect upon the material.to be'sifted, tthe terminology would equally apply to the substitution therefor of an inert gas to provide the desired sifting function.

I claim: V 1; In a sitting apparatus the combination-comprising a gas-containing chamberj a sifting screen placed across the chamber; an electrically responsive speaker providing a clo'sure for one end of said chamber and including a tric'al oscillator connected "to' the speaker for imparting oscillations of desired frequency to said vibratory diaand sealing off one end of the chamber; and anelectrically responsive'speaker positioned outside said membrane and her that'is transmitted thrcughsaid membrane.

bers; a sitting screen disposed'within each of the respective chambers; a pair of flexiblediaphragrns respectively disposed to enclose one end of each member'of said pair.

f of chambers; electromagnetic vibratory means'ar'ranged in each of said chambers.

4. in a sifting apparatus the combination comprising a pair of enclosed-communicating gas-containing chambers;

including a flexible diaphragm disposed rto encloser one electromagnetic, vibratory means arranged to oscillateea'ch pair in one direction does not etIectiVeIyprovide the de- 0f said respective .diaphragms; a common electrical cir- @siredoperation; The air should be supplied in re'cipro- 3 calor oscillating rnt'ion, in order to permit; thelighter i ranged "-insaiid circuit for alternate relative oscillation the respective;diaphragins'; whereby oscillationsof each diaphragm alternately create an oscillating gaseous co 5 within each of said'chambersg V provisionjmust be made for either protecting the speaker diaphragm or the protectivemembrane, orgbothpirom an accumulation of sifted particles settling'thereon; Such accumulation tends to' dampen the vibration and adversely .obtainedjby a reciprocably moving column-of air; For it instance, in cases where atmospheric air-may have an.

vibratory diaphragm arrangedto impart'anoscillat'ing mo- V tion to the gaseous colurnn'injsaidfchamber; and anclecapparatus the Tcoinbin'ation comprising 1 a an air chamber; a sifting screen disposed within thev including a flexible, osciilatory diaphragm disposed across ,the end of the chamber, whereby oscillations of the dia-i phragm create an oscillating air column Within said ch-amto oscillatecach of said respectivctdiaphragms in alter- 'nate phase relationship; whereby oscillations of each dia-i phragm alternately create an oscillating 'gas column witha sitting screen disposed within each of the respective 3 chambers; a pair of electrically responsive speakers each cuit for said speak'ers, said electromagnetic means ar i 5. In a sifting apparatus the combination comprising a pair of substantially parallel air chambers; a sifting screen disposed within each of the respective chambers; a transversely arranged air chamber communicating with one end of a respective one of said pair of air chambers to provide an enclosed continuous chamber therewith; a pair of flexible diaphragrns respectively disposed to enclose the opposite ends of each member of said pair of air chambers; electromagnetic vibratory means arranged to oscillate each of said respective diaphragms in alternate phase relationship; whereby oscillations of each diaphragm alternately create an oscillating air column within the enclosed system provided by said pair of chambers and said communicating air chamber.

6. In a sifting apparatus the combination comprising a pair of substantially parallel air chambers; a sifting screen disposed Within each of the respective chambers; a transversely arranged air chamber communicating With one end of a respective one of said pair of air chambers to provide an enclosed continuous chatnber therewith; a pair of electrically responsive speakers each providing a closure for the opposite end of a respective chamber and each including a vibratory diaphragm and electromagnetic means arranged to oscillate each of said respective diaphragms; a common electrical circuit for said speakers, said electromagnetic means arranged in said circuit for alternate relative oscillation of the respective I 10 diaphragms; whereby oscillations of each diaphragm al ternately create an oscillating air column Within the enclosed system provided by said pair of chambers and said communicating air chamber.

7. in a sitting apparatus the combination comprising: a gas containing chamber; a sifting screen placed across the chamber to intercept and have gas vibrating longitudinally Within said chamber pass back and forth through such screen; a vibratory diaphragm enclosing an end of said chamber whereby oscillations of the diaphragm will create longitudinal vibrations of gas in said chamber that are of a Width substantially coextensive with that of the diaphragm; and electrical vibratory means associated with said diaphragm to oscillate the same in response to the vibratory means at frequencies greater than 25 cycles per second.

References Cited in the file of this patent UNITED STATES PATENTS 796,553 Baugh Aug. 8, 1905 1,729,579 Hayes Sept. 24, 1929 2,283,051 Fuller May 12, 1942 2,312,712 Hartline Mar. 2, 1943 2,787,444 Skarstrom Apr. 2, 1957 FOREIGN PATENTS 475,352 Great Britain Nov. 20, 1936 

